Catalytic Hydrogenation of Cyclohexene 3

JOURNAL OF CATSLYSIS 52, 462-471 (1978)

Catalytic Hydrogenation of Cyclohexene 3. Gus-Phase and Liquid-Phase Reaction on Supported Palladium

E. E. GONZO AND M. BOUDART’

Department of C?Lemical Engineering, Stanford University, Stanford, California 94305 Received September 22, 1977; revised January 16, 1978

The hydrogenation of cyclohexene was studied in both gas and liquid phases on six different palladium catalysts supported on silica gel and alumina between 264 and 308 K at hydrogen pressures from 4 to 89 kPa, cyclohexene partial pressures from 0.46 to 1.5 kPa, and cyclohexene concentrations from 0.15 to 3.0 M. Under these conditions the turnover frequency is independent of particle size (1.5 to 10 nm) in both liquid and gas phases, of the nature of the support, and of the nature of nonaromatic solvents (methanol, n-heptane, ethyl acetate, and cyclohexene). The order of reaction was one-half with respect to hydrogen for the reaction in both liquid and gas phases and zero with respect to cyclohexene partial pressure (gas phase) or with respect to cyclohexene molar concent’ration (liquid phase). At the same temperature and hydrogen pressure, the turnover frequency was about twice as high in the gas phase as in the liquid phase.

INTRODUCTION for the hydrogenation of cyclic olefins. In The mechanism of the hydrogenation of particular, with supported metals, it is of alkenes on metal catalysts has been the interest to determine the effect of metal subject of many investigations (1). In 1934, particle size on turnover frequency. We Polanyi and Horiuti (2) proposed a mecha- have reported recently the lack of such an nism for the hydrogenation of ethylene on effect for the hydrogenation of cyclohexene, the surface of metallic nickel. This mecha- in both gas (10) and liquid phases (11) on nism still appears to give a good representa- different supported platinum catalysts. The tion of many aspects of catalytic hydro- present paper reports similar work on genation and related reactions on metals of supported palladium. Among previous in- group VIII, particularly nickel, platinum, vestigations of the hydrogenation of cyclo- palladium, and rhodium. hexene on supported palladium catalysts Information obtained from studies using (12-16) only one (12) gives the surface area isotopic hydrogen both in exchange reaction of palladium metal. At any rate, earlier with saturated hydrocarbons (3-5) and in results will be discussed below together the hydrogenation of alkenes (6-9) has with our own results. contributed particularly to the present state of understanding of these reactions. REACTION IN GAS PHASE On the other hand, kinetic data, especially Expev+imental Apparatus area1 rates, are relatively scarce, especially The gas-phase reaction of cyclohexene 1 To whom queries concerning this paper should with hydrogen was studied in a Pyrex be sent. batch-recirculation system (17) under con-

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OOZl-9517/78/0523-0462$02.00/O Copyright Q 1978 by Academic Press. Inc. All righta of reproduction in rtny form reserved. CATALYTIC HYDROGENATION OF CYCLOHEXENE 463

ditions designed Qo obtain kinetic data free TABLE 1 from all transport influences. The reactor Koros and Nowak Criterion0 was a glasschamber 25 mm in diameter and about 15 mm high wit,h a medium-grade Catalyst Dispersion Turnover frequency fritted disk to support the catalyst in G) (set-I) amounts between 3.5 and 10 mg. A glass 273 K 283 K coil above the rcact,or served to preheat or precool the reaction gases. The reactor was 3.7574Pd/SiOz 55 0.72 1.36 immersed in a Dewar t’hrough which was 0.57y0 Pd/SiOl 60 0.76 I .42 flowed a 50% v,‘v solution of water and o See Ref. (25). Conditions : hydrogen pressure, ethylene glycol, from a t,hermost,at,ed bath. 10.64 kPa; cyclohexene pressure, 1.0 kPa. The temperature was measured with a thermocouple inserted int,o a well extending was used after passaget,hrough a glass coil through the reactor wall to a point just at 75 K. above the center of the fritted disk. The gases were circulated by a noncont,aminat- ing gas-recirculating pump (18). A flow rate of 960 cm3 min’ was used in a syst’em The amount of palladium in the six sup- of 2.34 liters total volume, and 23-cm3 ported catalysts described below was deter- samples were withdrawn at approximately mined by atomic absorption. It is reported S-min intervals for chromatographic anal- as a weight percentage. A catalyst from ysis. A bypass in t,he system made it Engelhard Industries, 4.557 Pd/A1203, possible to int,roduce and mix the reactant was reduced at 773 K in hydrogen, out- gases in the loop while t)he cat,alyst in the gassed at that temperature, and cooled react,or was still in pretreatment stages. In down to room temperature. Then it was order to facilitate sampling, another bypass exposed to air prior to use. The dispersion of permitted samples to be taken without palladium was determined to be 0.21 by interruption of the flow over t#he catalyst. the titration method of Benson et al. (19) The samples were obtained with a Varian- which was used for all other catalysts as Aerograph X.1-210 six-way sample valve well. Values of dispersion are given in and were analyzed in a Ovarian-Aerograph Table 2. A 2.S4% Pd/SiOz catalyst was A-90-I’ chromatograph equipped with a prepared by impregnation using a mea- 3-m column of 70y0 Chromosorb P and sured amount of PdClz dissolved in a small 30% dibut,ylphtalate which operated at quantity of concentrated HCl. The result- 373 K. ing solution was evaporated to boil off Matheson, Coleman and Bell chromato- excess acid and distilled water was added quality cyclohexene was used after purifica- to obtain the required amount of salt tion through an activated alumina column solution to just reach the point of incipient in order to remove peroxides and traces of wetness of the support (Davison silica gel, oxygen. The A1203 column was breated Grade 02). The catalyst was calcined for 4 h with flowing He at 693 K for 1 h, and a in air at 723 II and reduced in hydrogen at bulb with cyclohexene was then attached 773 Ii. Four catalysts, 0.570/, l’d/SiOz, to the column: after which a freeze-pump- 1.39% Pd/SiOz, 1.45y0 I’d/SiOz, and 3.75% thaw technique was used to bring the l-‘d/SiOs, were prepared by cation ex- cyclohexene t,o t,he storage bulb. The change (20, 21). The first step of prepara- hydrogen, 99.932y0 pure, was further tion was to exchange NHJ+ from a 1: 1 purified by diffusion t#hrough a Milton Roy diluted N&OH solution to the surface of hydrogen diffuser. Helium (99.9959/‘0pure) the silicn gel. The silica gel was 50/100- 464 GONZO AND BOUDART

maintain the catalyst saturated with hydrogen (adsorbed and absorbed) and the rest of the system was out’gassedfor 0.5 h. As the catalyst was being reduced, the reactant gases were added and circulated through t’he system, bypassing the reactor. The gases were introduced in the following order: cyclohexene (0.46 to 1.46 kPa), hydrogen (4.52 to 85.3 kPa), and helium (to balance the system at atmospheric total pressure). The circulation of the thermostatic bath solution was started. When the reactor had reached the reaction tempera- 1 I I 1 ture, the reactants were admitted to the 012345 catalyst chamber. The first sample for t/lo3 s chromatographic analysis was taken after 3 min, allowing for thorough mixing within FIG. 1. Hydrogenation of cyclohexene : fractional conversion f vemus time t. Catalyst: 4.88y0 Pd/ the recirculating system loop. The sub- A120a; dispersion : 21% ; Temperature: 273 K; sequent samples were taken at time in- ~HZ/~cyolohexe,,e : 100. tervals ranging from 480 t,o 600 set, depend- ing upon the reaction rate. The react,ion mesh Davison Grade 62. The second step rate was taken as t,he average between the was the cation exchange between the initial slopes of the plot of quantity of Pd(NHs)J2+ ion from the solution and the product (cyclohexane) and quantity of surface NH4 ions. The Pd(NHJ4C12 solu- reactant (cyclohexene) versus Gme. tion was prepared by dissolving PdC12 in concentrated NH?OH solution at 348 K Results and then diluting to obtain a 0.01 M The reaction rate is given as turnover solution. The palladium salt solution was frequency (22) defined as the average num- slowly added to a slurry of the support in ber of molecules of cyclohexane produced 1: 1 dilute NH,OH solution. The slurry was stirred for 2 h at 348 K. The next step of TABLE 2 preparation was to filter the slurry. The filtrate was washed with distilled water and Effect of Palladium Dispersiona dried in vacua in an oven overnight at room Condition Gas phase Liquid phase temperature. The catalysts were calcined Temperature (K) 273 308 in air at 723 K for 4 h and reduced at 773 K Hydrogen pressure 10.84 kPa Atmospheric in hydrogen. Cyclohexene pressure or concentration 1 .O kPa 0.24 M in cyclohoxsne

Procedure Loading Support Dispersion Turnover frequency (%) (%) (se0)

The standard pretreatment of the cata- GM Liquid lysts listed above consisted of a 3-h out- phi% phase gassing at room temperature (pressure : 2.84 SiOt 11 0.58 6.54 1.3 x 1O-3 Pa), followed by l-h reduction 4.88 A1203 21 0.97 7.95 3.75 SiOz 55 0.72 6.54 in flowing hydrogen (75 cm3 min-‘) at room 0.57 SiOz 60 0.76 6.47 temperature. After the reduction, the two 1.39 SiOz 70 0.80 8.25 1.45 SiO* 76 0.90 7.96 stopcocks of the reactor were closed to CATBLYTIC HYDROGENATION OF CYCLOHEXENE 465 and cyclohexene consumed per surface TABLE 3 palladium atom per second (LV set-I) under Hydrogenation of Cyclohexene is Zero Order in the conditions indicated. The rate was zero Hydrocarbon Molar Concentration& with respect 60 cyclohexene (Fig. 1) and Cyclohexene Turnover one-half order wit,h respect to hydrogen concentration frequency (Fig. 2). (dl) (see-I) The absence of both internal and external heat and mass transfer limitation are indi- 3.01 7.79 cat,ed by the values of -1: shown in Table 1. 1.88 7.77 1.10 7.8.5 The absence of any significant change in .\: 0.69 7.69 at two different temperat)ures, with two 0.24 7.7:s different catalysts that differ sevenfold in 0.15 7.74 met’al loading but have similar dispersion, is a convincing test that the reaction rates a Conditions : hydrogen pressure, atmospheric ; temperature, 308 K; catalyst, 4.887, Pd/Als03; are correctly measured (23, 24). L) = 0.21; solvent, cyclohexane. So effect of metal dispersion on the reaction rat,e was found as shown in Table 2 so that the hydrogenation of cyclohexene REACTION IN LIQUID PHASE on supported palladium can be regarded as a st’ructure-insensitive reaction (25-27). The temperature dependence of t’he rate The apparat’us described earlier (11) and constant k, i.e., t’he apparent act,ivat#ion shown in Fig. 4 was used to study t)he energy E:,, was found to be 38.90 kJ mol-’ liquid-phase hydrogenation of cyclohexene. (Fig. 3) and the preexponential factor was The vacuum was provided by a roughing S.45 X 10F set-’ kI’a-+. pump and a mercury diffusion pump connected to the system via a trap at 78 K. The reaction t,emperature was cont,rolled r- by pumping water from a thermostatic bath, through the reactor jacket. The water flow rate was controlled ‘with a variable transformer. The temperature of the reactor mixture was measured by a t,hermocouple inserted into a well through the reactor wall, which dipped into the reactants. The reactor (IS0 cm3, total capacit’y) was con- nect’ed to the apparatus via a standard taper joint and a flexible Teflon t’ube. It also had an arm with a Teflon stopcock and a rubber serum cap through which the reactant (cyclohexene) and the solvents were introduced. The reactor was agitated at a frequency of 3.50 min-l. The total pressure in the system was controlled by a p1/2/k Pa “’ constant-pressure device (24, 28). Hydro- grn was passed through an ICngelhard FIG. 2. Effect of hydrogen partial pressure on the turnover frequency for the gas-phase hydrogenation Deoxo purifier and a 18X molecular sieve of cyclohexene. Catalyst : 4.88’% Pd/AlzOa; dis- trap. Helium, used in the treat,ment’ and persion: 21’%. (0) 264 K, (0) 273 K, (A) 283 K. storage of cyclohexenc, was passed through 466 GONZO AND BOUDART

opened and the reactor and catalyst were 0.6 evacuated for 10 min while the Teflon stopcock on the reactor was open. Stopcock 6 0.5 was then closed, the liquid nitrogen Dewar on trap B was removed, and the trap was ? 0.4 evacuated for 15 min in order to remove 5 oxygen and water vapor collected in the ‘; 0.3 trap. The Dewar with liquid nitrogen was < placed on the trap and the whole apparatus 2% was then evacuated (stopcock 6 and Teflon stopcock on the reactor were open while 0.2 the catalyst outgassed for 2 h at room temperature pressure, 1.3 X lop2 Pa). After turning stopcock 1, to stop evacuation, 3.4 3.5 3.6 3.7 3.8 3.9 hydrogen was admitted into the apparatus. IO3 K/T The catalyst was kept in the presence of hydrogen at room temperature for 1 h. At FIG. 3. Arrheniusplot for the gas-phasehydro- this time, stopcocks 4 and 5 were opened genationof cyclohexene.Catalyst : 4.88y0Pd/AlzOs; to fill up the burette with hydrogen while dispersion: 217$. the pressure was controlled by observing the Hg manometer. a trap at 723 K containing copper turnings and then through a 13X molecular sieve The hydrogen was introduced until the total pressure was near to the reaction trap at 78 K. The cyclohexene was purified as described above and elsewhere (11, $4) pressure. The water from the thermostatic from the hydroperoxides. The solvents bath at the reaction temperature was (spectroquality) were used without any circulated around the reactor. The solvent addit,ional purification. was now introduced, through the serum cap The catalysts used were the same as and the Teflon stopcock, with a 15-cm-long stainless steel needle and glass syringe. those described above. The reaction was carried out with catalyst particles smaller Agitation of the reactor was started to make it easy for the solvent Do achieve a than 44 pm (greater than 325 mesh). It was found that, with particle sizes greater steady temperature and saturation in hy- than 100 pm (lower than 200 mesh), mass drogen. After 10 min, agitation was transport effects play a dominant role stopped, purified cyclohexene was added, and hydrogen was introduced until the (11, $4). total pressure reached the desired value. Procedure Meanwhile the manostat controlling the total pressure was switched on. Stopcock 1 The catalyst was dried in vacua in an was turned to isolate the apparatus from oven overnight at 383 K, weighed, and the arm connected with trap C and the placed in the reactor which was connected vacuum system. After the initial level of to the apparatus (Fig. 4). The temperature the Hg in the burett,e had been noted, the of traps A and B was at 78 K whereas the 13X molecular sieve (C) was at room agitation and the timer were started temperature. Stopcocks 3, 4, and 5 were simultaneously. The reaction rate, at con- kept closed. The line connecting trap C stant pressure, was calculated by taking with the rest of the apparatus was isolated the initial slope of the plot of the volume from t,he vacuum line. Stopcock 6 was of hydrogen consumed versus time. CATALYTIC HYDROGENATION OF CYCLOHEXENE 467

1 VACUUM GAUGE

TRAP(B) Au TRAP I II DEOXO PURIFIER P II IIHg BURETTE TER TURNINGS

t: PRESSURE DEVICE

FIG. 4. &lain apparatus for liquid-phase hydrogenation.

Results and it was found to be 31.51 kJ mol-‘. The preexponential factor was 1.96 X lo5 set-l The rates are given as turnover frequency kPa-4. The six catalyst supports listed in as previously defined. The initial rates of Table 2, with different modes of preparation reaction of cyclohexene with hydrogen and dispersion varying from 11 to 76%, have been corrected for the changes in the were used. The average palladium particle partial pressure of hydrogen in t,he reaction size of the catalysts varied from 1 to 8 nm flask result’ing from the solvent vapor (21). The t,urnover frequency was found to pressure. be independent of palladium dispersion As in the gas phase, we used the Koros- (Table 2), just’ like in t’he gas phase. Nowak criterion to examine the gradient- less behavior of the reacting system. For two catalysts with palladium loadings of 0.57 and 3.75y0 and palladium dispersions / of 60 and 55%, respectively, values of AV 0 were 6.47 and 6.54 se@ at 308 K. Corre- E 10 sponding values were 2.70 and 2.79 se+ ?- L-- at 2SS K. Thus our kinetic data are free from all transport influences and besides the catalyst was perfectly suspended in 5 the liquid mixture (Fig. 5). The order was one-half with respect to J hydrogen pressure (Fig. 6). The dependence IO 15 of the reaction rate on substrat’e concen- mg catalys it tration corresponds to zero order. The hydrogenation of cyclohexcnc was also FIG. 5. Reaction rate r as a function of the weight of catalyst for the liquid-phase hydrogenation of carried out in different solve&s (Table 5). cyclohexene. Catalyst : 4.8870 Pd/AltOa ; dispersion : The apparent activation energy El was 21%; temperature: 308 K; solvent: cyclohexnne; obtained from an hrrhenius plot (Fig. 7) hydrogen pressure : atmospheric. 46s GONZO AND BOUDART

k3 *R*+H*+RH*+%* (3 k-3 N/s-’ ,’ 1 kr i I RH*+H*+RH2+2* (4) It has been shown that cycloalkanes do not dissociatively chemisorb on palladium catalysts under our conditions (3, 5), hence the addition of the second hydrogen in step 4 can be regarded as irreversible. But the formation of large amounts of ex- I I I I III1 30 40 50 60 70 8090 changed alkene when deuterium is used is enough evidence that addition of the first p/kPa hydrogen and olefin adsorption are highly FIG. 6. Hydrogenation of cyclohexene: effect of reversible (5, 6, 13). The appearance of a hydrogen pressure on turnover frequency. Catalyst: small amount of HD in the gas phase shows 4.887, Pd/AlzOa; dispersion : 21 ye; temperature : t’hat hydrogen adsorption is also reversible 308 K ; solvent : cyclohexane. (6, 1s). We now assume that hydrogen chemi- DISCUSSION sorption, step 2, is in equilibrium with constant lcz and that the most abundant Using the Koros-Nowak criterion, we surface intermediate is the half- hydro- have shown that our catalytic data, in both genated state, RH*, covering almost en- liquid and gas phases, are free from transport influences. With confidence in our rate data, we think the salient result of the work is the observation that, with 8 values of palladium dispersion from 11 to 76% corresponding to average particle size 7 from 1 to 8 nm, the turnover frequency is N/S-’ independent of the palladium particle size 6 within experimental error, in both liquid and gas phases (Table 2). Thus the hydrogenation of cyclohexene on palladium is structure insensitive as it was found to be on platinum (10, 11). In addition the catalytic activity of the metal is independent of t’he nature of the support when silica gel and alumina are used as supports. According to Polanyi and Horiuti (2) the addition of hydrogen to an olefin is described by the following elementary process. 3.2 3.3 3.4 3.5 kl IO K/T Ii + 2* $ *R* (1) k-1 FIG. 7. Arrhenius plot for the liquid-phase hydro- kz genation of cyclohexene. Catalyst : 4.88’3e Pd/AlsOa; dispersion: 21%; hydrogen pressure: 101.1 kPa; Ha+Z*atH* (2) k-1 solvent : cyclohexane. CATALYTIC HYDROGENBTION OF CYCLOIIEXENE 469 tirely t’lic :tvtLilable surface. Thcsc asaump- TABLE 5 tions are compatible with t’he small amount Turnover Frequencyfor CyclohexeacIIydro- of HD found in the gas phase, the appear- genationin Liquid and Gas Phasesa ance of deuterium in the olefin when Turnover frequency deuterium is used, and the zero order of (see-I) reaction wit’h respect t,o hydrocarbon. With the additional assumptjion that Gas Liquid chemisorbed hydrogen covers only a small phase phitse fraction of the free sites, we have (a) Temperature: 283K (L) ‘v (RH*) and (RH*) >> (*) >> (H*), Hydrogenpressure : 53.2kPa 4.0 1.9 where (L) is the number of act.ive sites per (h) Temperature: 288K unit area of metal and (RH*), (*), and Hydrogenpressure : (H*) are the concentrations of the half- 53.2kPa 5.32 2.40 hydrogenated state, the free active sites, a Conditions:catalyst,, 4.88’]6Pd/A1203; D = 0.21. and the atomic hydrogen adsorbed, respectively. Under these conditions the rate r of vents such as benzene and toluene the reaction is t’hat of step 4 and is given by reaction rate depended strongly on the relative concentration of the cyclohexene r = r4 = (L)k,k,+(H,):, and benzene (Table 4). The more basic in agreement with the kinetic results. solvent xylene competes more strongly for No solvent effect was found with non- the surface than benzene. Neither benzene aromatic solvents, polar or nonpolar, which nor xylene are hydrogenated under our did not compete for the sit,es with the experimentSal conditions. cyclohexene or with the hydrogen shown In Table 5 we report the value of the by our results with met’hanol, ethyl acet’ate, turnover frequency for the hydrogenation cyclohexane, and n-heptane, and also by of cyclohexene in liquid and gas phases, t,he results of others with acetone and under the same conditions of temperat,ure n-hexane (14, 15). But with aromatic sol- and hydrogen pressure. The turnover frequency for gas-phase hydrogenat’ion is TABLE 4 twice the turnover frequency for liquid- Cyclohexene Hydrogenation in Different SolvenbsB phase hydrogenation. Although t)he fraction of the surface Solvent Turnover covered by the half-hydrogenated stat,e frequency (see-l) appears t,o be almost unit,y, increasing the cyclohexene concent’ration should increase Methanol 7.85 the surface coverage of the half-hydro- n-Heptane 7.98 genated state with a reduction in the avail- Ethyl acetate 7.93 able surface area for hydrogen chemisorp- Cyclohexane 7.80 Benzene 0.58 tion. In the liquid phase, the cyclohexene concentjrat’ion was from 0.2 to 3 M, but in Benzene” 2.78 the gas phase the largest. concentration was Xyleneb 2.49 only about ci X lop4 M. Thus it appears that the difference between the turnover a Conditions: hydrogen pressure, atmospheric; t,emperature,308 K ; catalyst, 4.88y0 Pd/AlrOs; frequency in liquid and gas phases is due D = 0.21 ; cyclohexeneconcentration : 0.24 M. to the difference in the fraction of the b Cyclohexeneconcentration : 3.01 J1. surface covered by the half-hydrogenated 470 GONZO AND BOUDART shtc, bccauxc of the ditIcrunt cyclohexenc given. They found that the m&ion rutc concentrations used in liquid and gas does not depend on solvent composition. phases. We are not aware of any other The same authors (15) carried out the clear-cut quantitative comparison between reaction of hydrogenation of a mixture of catalytic rates in bot.h gas and liquid cyclohexene and acetone on palladium. phases. They found that the rate of cyclohexene Finally, our data for the hydrogenation hydrogenation does not depend on solvent of cyclohexene on palladium catalysts will composition and that acet,one is hydrogen- be compared to those of others. ated to a negligible extent. Our results with The hydrogenation rates in liquid phase nonaromatic solvents are in agreement. on palladium black catalysts were reported with those of Druz et al. (15). by Spitsyn et al. (12). The value of the turnover frequency (assumption : 1.2 X lOI ACKNOWLEDGMENT sites cm-z) are IV = 1 se+ for a palladium black prepared by the Zelinskii method and The preparationof the manuscriptwas supported N = 0.7 se+ for the palladium black by NSF ENG 74-22234 AOl. One of us (EEG) car- prepared by a method involving radiation, ried out the work while on leave from the University of Salta while supported in part by the government both at atmospheric hydrogen pressure and of Argentina. at 293 K. The turnover frequency obtained by us for the same experimental conditions is 4.1 se+, which is four times greater than REFERENCES that obtained by Spitsyn et al. (12). These 1. Bond, G. C., and Wells, P. B., Advan. Cutal. authors also report reaction orders in com- Relat. Subj. 15, 91 (1964). plete agreement with ours, with respect to 2. Polanyi, M., and Horiuti, J., Trans. Faraday the kinetic order of both cyclohexene and Sot. 30, 1164 (1934). s. Anderson, J. R., and Kemball, C., PTOC. Roy. hydrogen. Sot. (London) 226A, 472 (1954). Hussey and Nowack (IS) studied the 4. Schrage, K., and Burwell, R. L., Jr., J. Amer. liquid-phase hydrogenation of cyclohexene Chem. Sot. 88, 4549 (1966). on two l’d/Al,Oe catalysts. Unfortunately, 5. Burwell, R. L., Jr., Act. Chem. Res. 2,289 (1969). as no palladium dispersion was reported, a 6. Bond, G. C., and Winterbottom, J. M., Trans. Faraduy Sot. 65, 2779 (1969). turnover frequency could not be obtained 7. Bond, G. C., Philipson, J. J., Wells, P. B., and from their data. The reaction orders found Winterbottom, J. M., Trans. Faraday Sot. by Hussey and Nowack were 0.2 to 0.3 62, 443 (1966). with respect to cyclohexene and 0.45 f 0.05 8. Weitkamp, A. W., J. Cutal. 6, 431 (1966). with respect to hydrogen pressure. But 9. Smith, G. V., and Swoap, J. It., J. Org. Chem. Hussey and Nowack state that their rates 31, 3904 (1966). reflect an intrinsic zero-order process upon 10. Segal, E., Madon, It. J., and Boudart, M., J. Catal., in press. which a first-order hydrogen pore-diffusion 11. Madon, It. J., O’Connell, J., and Boudart, M., process is superimposed. Hussey and submitted. Nowack used 150- to 200-mesh particles, 12. Spitsyn, V. I., Balandin, A. A., and Barsova, and, with this size of catalyst particles, the L. I., Russ. J. Phys. Chem. 42, 184 (1968). rates of liquid-phase hydrogenation of IS. Hussey, A. S., and Nowack, G. P., J. Org. Chem. cyclohexene on platinum are indeed in- 34, 439 (1969). fluenced by intraparticle diffusion (24). 14. Druz, V. A., Sokol’skii, D. V., and Dauletov, B., K&et. Katal. 9, 43 (1968). Druz et al. (14) studied the hydrogena- 16. Druz, V. A., Sokol’skii, D. V., and Dauletov, B., tion of cyclohexene in mixtures of methanol K&et. K&a/. 8, 914 (1967). and n-hexane on palladium black. The 16. Reshetnikov, S. M., and Sokol’skaya, A. M., surface area of the metal catalyst is not Kinet. Katul. 7, 279 (1966). CATALYTIC HYDROGENATION OF CYCLOHESENE 4’71

17. Tenrkin, &I. I., Ziinet. Kutul. 3, 509 (1962). 2/t. Madon, It. J., Ph.D. l~isserlalion, Stanford 18. HBIISOII, F. V., and Berw~, J. I<., J. Ctctul. 31, University (1975). 471 (1973). 26. Boronin, V. S., Nikulina, V. S., and Poltorak, 19. Benson, J. E., Hwang, H. S., and Boudart, &I., 0. M., Russ. J. Phys. Chem. 37, 626 (1963). J. Catal. 30, 146 (1973). 2G. Boudart, M., Advan. Catal. Relat. Subj. 20, 153 20. Sinfelt, J. H., J. Catal. 29, 308 (1973). (1969). 27. Boudart, M., in “Proceedings, VIth Inter- 22. Hwang, H. S., Ph.D. Dissertation, Stanford national Congress on Catalysis,” (Bond, Cr. University (1975). C., Wells, P. B., and Tompkins, F. C., Eds.), 22. Boudart,, M., AZChE J. 18, 465 (1972). Vol. 1, p. 1. Chemical Society, London, 1977. 23. Koros, It. M., and Nowak, E. J., Chenz. Eny. 28. Boudart, M., Aldag, A. W., and Vannice, M. A., Sci. 22, 470 (1967). J. Catal. 18, 46 (1970).